Graphene nanoribbon

Atomic Force Microscopy (AFM) images of graphene nanoribbons having periodic width and boron doping pattern. The polymerization reaction used for their synthesis is shown on top.[1]

Graphene nanoribbons (GNRs, also called nano-graphene ribbons or nano-graphite ribbons) are strips of graphene with width less than 50 nm. Graphene ribbons were introduced as a theoretical model by Mitsutaka Fujita and coauthors to examine the edge and nanoscale size effect in graphene.[2][3][4]

Large quantities of width-controlled GNRs can be produced via graphite nanotomy,[5] where applying a sharp diamond knife on graphite produces graphite nanoblocks, which can then be exfoliated to produce GNRs. GNRs can also be produced by "unzipping" or axially cutting nanotubes.[6] In one such method multi-walled carbon nanotubes were unzipped in solution by action of potassium permanganate and sulfuric acid.[7] In another method GNRs were produced by plasma etching of nanotubes partly embedded in a polymer film.[8] More recently, graphene nanoribbons were grown onto silicon carbide (SiC) substrates using ion implantation followed by vacuum or laser annealing.[9][10][11] The latter technique allows any pattern to be written on SiC substrates with 5 nm precision.[12]

GNRs were grown on the edges of three-dimensional structures etched into silicon carbide wafers. When the wafers are heated to approximately 1,000 °C (1,270 K; 1,830 °F), silicon is preferentially driven off along the edges, forming nanoribbons whose structure is determined by the pattern of the three-dimensional surface. The ribbons had perfectly smooth edges, annealed by the fabrication process. Electron mobility measurements surpassing one million correspond to a sheet resistance of one ohm per square— two orders of magnitude lower than in two-dimensional graphene.[13]

Nanoribbons narrower than 10 nm grown on a germanium wafer act like semiconductors, exhibiting a band gap. Inside a reaction chamber, using chemical vapor deposition, methane is used to deposit hydrocarbons on the wafer surface, where they react with each other to produce long, smooth-edged ribbons. The ribbons were used to create prototype transistors.[14] At a very slow growth rate, the graphene crystals naturally grow into long nanoribbons on a specific germanium crystal facet. By controlling the growth rate and growth time, the researchers achieved control over the nanoribbon width.[15]

Recently, researchers from SIMIT(Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences) reported on a strategy to grow graphene nanoribbons with controlled widths and smooth edges directly onto dielectric hexagonal boron nitride (h-BN) substrates.[16] The team use nickel nanoparticles to etch monolayer-deep, nanometre-wide trenches into h-BN, and subsequently fill them with graphene using chemical vapour deposition. Modifying the etching parameters allows the width of the trench to be tuned to less than 10 nm, and the resulting sub-10-nm ribbons display bandgaps of almost 0.5 eV. Integrating these nanoribbons into field effect transistor devices reveals on–off ratios of greater than 104 at room temperature, as well as high carrier mobilities of ~750 cm2 V−1 s−1.

A bottom-up approach was investigated.[17][18] In 2017 dry contact transfer was used to press a fiberglass applicator coated with a powder of atomically precise graphene nanoribbons on a hydrogen-passivated Si(100) surface under vacuum. 80 of 115 GNRs visibly obscured the substrate lattice with an average apparent height of 0.30 nm. The GNRs do not align to the Si lattice, indicating a weak coupling. The average bandgap over 21 GNRs was 2.85 eV with a standard deviation of 0.13 eV.[19]

The method unintentionally overlapped some nanoribbons, allowing the study of multilayer GNRs. Such overlaps could be formed deliberately by manipulation with a scanning tunneling microscope. Hydrogen depassivation left no band-gap. Covalent bonds between the Si surface and the GNR leads to metallic behavior. The Si surface atoms move outward, and the GNR changes from flat to distorted, with some C atoms moving in toward the Si surface.[19]

The electronic states of GNRs largely depend on the edge structures (armchair or zigzag). In zigzag edges each successive edge segment is at the opposite angle to the previous. In armchair edges, each pair of segments is a 120/-120 degree rotation of the prior pair. Zigzag edges provide the edge localized state with non-bonding molecular orbitals near the Fermi energy. They are expected to have large changes in optical and electronic properties from quantization.

Calculations based on tight binding theory predict that zigzag GNRs are always metallic[contradictory] while armchairs can be either metallic or semiconducting, depending on their width. However, density functional theory (DFT) calculations show that armchair nanoribbons are semiconducting with an energy gap scaling with the inverse of the GNR width.[20] Experiments verified that energy gaps increase with decreasing GNR width.[21] Claims that graphene nanoribbons with controlled edge orientation have been fabricated by scanning tunneling microscope (STM) lithography[22] are highly controversial and some of the data might have been fabricated, as one of the authors has been recognized guilty of scientific misconduct by an official commission of the Max Planck Gesellschaft. Energy gaps up to 0.5 eV in a 2.5 nm wide armchair ribbon were reported.

Zigzag nanoribbons are semiconducting[contradictory] and present spin polarized edges. Their gap opens thanks to an unusual antiferromagnetic coupling between the magnetic moments at opposite edge carbon atoms. This gap size is inversely proportional to the ribbon width[23][24] and its behavior can be traced back to the spatial distribution properties of edge-state wave functions, and the mostly local character of the exchange interaction that originates the spin polarization. Therefore, the quantum confinement, inter-edge superexchange, and intra-edge direct exchange interactions in zigzag GNR are important for its magnetism and band gap. The edge magnetic moment and band gap of zigzag GNR are reversely proportional to the electron/hole concentration and they can be controlled by alkaline adatoms.[25]

Their 2D structure, high electrical and thermal conductivity and low noise also make GNRs a possible alternative to copper for integrated circuit interconnects. Research is exploring the creation of quantum dots by changing the width of GNRs at select points along the ribbon, creating quantum confinement.[26] Heterojunctions inside single graphene nanoribbons have been realized, among which structures that have been shown to function as tunnel barriers.

The earliest numerical results on the optical properties of graphene nanoribbons were obtained by Lin and Shyu in 2000.[31] The different selection rules for optical transitions in graphene nanoribobns with armchair and zigzag edges were reported. These results were supplemented by a comparative study of zigzag nanoribbons with single wall armchair carbon nanotubes by Hsu and Reichl in 2007.[32] It was demonstrated that selection rules in zigzag ribbons are different from those in carbon nanotube and the eigenstates in zigzag ribbons can be classified as either symmetric or antisymmetric. Also, it was predicted that edge states should play an important role in the optical absorption of zigzag nanoribbons. Optical transitions between the edge and bulk states should enrich the low-energy region (<3{\displaystyle <3} eV) of the absorption spectrum by strong absorption peaks. Analytical derivation of the numerically obtained selection rules was presented in 2011,.[33][34] The selection rule for the incident light polarized longitudinally to the zigzag ribbon axis is that ΔJ=J2−J1{\displaystyle \Delta J=J_{2}-J_{1}} is odd, where J1{\displaystyle J_{1}} and J2{\displaystyle J_{2}} number the energy bands, while for the perpendicular polarization ΔJ=J2−J1{\displaystyle \Delta J=J_{2}-J_{1}} is even. Intraband (intersubband) transitions between the conduction (valence) subbands are also allowed if ΔJ=J2−J1{\displaystyle \Delta J=J_{2}-J_{1}} is even.

Optical selection rules of zigzag graphene nanoribbons

For graphene nanoribbons with armchair edges the selection rule is ΔJ=J2−J1=0{\displaystyle \Delta J=J_{2}-J_{1}=0}. Similar to tubes transitions intersubband transitions are forbidden for armchair graphene nanoribbons. Despite different selection rules in single wall armchair carbon nanotubes and zigzag graphene nanoribbons a hidden correlation of the absorption peaks is predicted.[35] The correlation of the absorption peaks in tubes and ribbons should take place when the number of atoms in the tube unit cell Nt{\displaystyle N_{t}} is related to the number of atoms in the zigzag ribbon unit cell Nr{\displaystyle N_{r}} as follows: Nt=2Nr+4{\displaystyle N_{t}=2N_{r}+4}, which is so-called matching condition for the periodic and hard wall boundary conditions. The aforementioned results were obtained within the nearest-neighbor approximation of the tight-binding model neglecting the excitonic effects.

Graphene nanoribbons can be analyzed by scanning tunneling microscope, Raman spectroscopy,[40] and infrared spectroscopy.[41] However, analyses of graphene nanoribbon on substrates are difficult using infrared spectroscopy even with a Reflection Absorption Spectrometry method. Thus, a large quantity of graphene nanoribbon is necessary for infrared spectroscopy analyses.

Graphene nanoribbons and their oxidized counterparts called graphene oxide nanoribbons have been investigated as nano-fillers to improve the mechanical properties of polymeric nanocomposites. Increases in the mechanical properties of epoxy composites on loading of graphene nanoribbons were observed.[42] An increase in the mechanical properties of biodegradable polymeric nanocomposites of poly(propylene fumarate) at low weight percentage was achieved by loading of oxidized graphene nanoribbons, fabricated for bone tissue engineering applications.[43]